fix: Changing Pivot.SelectedItem should reflect on Pivot.SelectedIndex

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_Pivot.cs

@@ -92,6 +92,29 @@ public async Task Check_Changing_Header_Affects_UI()
 			headerItem.Content.Should().Be("New text");
 		}
 
+		[TestMethod]
+		[RunsOnUIThread]
+		public async Task Check_Changing_SelectedItem_Affects_SelectedIndex()
+		{
+			var pivotItem1 = new PivotItem { Header = "First" };
+			var pivotItem2 = new PivotItem { Header = "Second" };
+			var SUT = new Pivot
+			{
+				Items = { pivotItem1, pivotItem2 },
+			};
+
+			WindowHelper.WindowContent = SUT;
+			await WindowHelper.WaitForIdle();
+
+			SUT.SelectedIndex.Should().Be(0);
+			SUT.SelectedItem.Should().Be(pivotItem1);
+
+			SUT.SelectedItem = pivotItem2;
+
+			SUT.SelectedIndex.Should().Be(1);
+			SUT.SelectedItem.Should().Be(pivotItem2);
+		}
+
 		private class MyContext
 		{
 			public MyContext()

src/Uno.UI/UI/Xaml/Controls/Pivot/Pivot.cs

@@ -337,8 +337,17 @@ private static void OnTitlePropertyChanged(DependencyObject dependencyObject, De
 
 		private void OnSelectedItemPropertyChanged(object oldValue, object newValue)
 		{
-			var removedItems = oldValue == null ? new object[0] : new[] { oldValue };
-			var addedItems = newValue == null ? new object[0] : new[] { newValue };
+			var removedItems = oldValue == null ? Array.Empty<object>() : new[] { oldValue };
+			var addedItems = newValue == null ? Array.Empty<object>() : new[] { newValue };
+
+			if (newValue is PivotItem newItem && newItem.PivotHeaderItem is PivotHeaderItem headerItem)
+			{
+				var newIndex = _staticHeader.Children.IndexOf(headerItem);
+				if (newIndex > -1)
+				{
+					SelectedIndex = newIndex;
+				}
+			}
 
 			OnSelectedItemChangedPartial(oldValue, newValue);